[R-390] Varying gain [WAS: Filters] and archives

[Norman Ryan]

Hi,

Fifty ohms across the IF mechanical filter coils is nominal.  Resistances measured can vary up to five ohms either way, so you're good there.

Resistance to the shell (ground) should read infinity.  I'm not sure 34 megohms is inconsequential.  I forget how the IF deck performs when there is ohms leakage to ground.  Perhaps someone can chime in here.

Rotate the bandwidth switch so that contact is not made with the filter coils when you are testing so as to get a correct reading.  A basic digital VOM works for me, and I'm sure an older analog meter wouldn't draw damaging current across the coils.

You probably should avoid opening up a more or less working filter.  Do that on a failed unit instead!  Here is a link to an excellent article on mechanical filter restoration:
http://www.delphelectronics.co.uk/filter390a/
I think this article should be required reading and saved to the hard drive -- as well as bookmarked in the unlucky event that the link may not work some time in the future.

73, all,
Norman

--- On Tue, 2/5/13, quartz55 <quartz55 at hughes.net> wrote in part:

Another filter question.  I have pulled the input circuits off the 2 and 4K Collins filters and at least the input seems to measure pretty good on the Fluke, around 50 ohms across the terminals, and about 34Meg to ground.  I haven't checked from in to out yet, but should get to that today.  Seems like the filters measure good with the ohmmeter, but don't act right, and the rest of the circuit components seem fine.  Need to check the switching though.  I'm not adverse to pulling the filters apart to see if I can work on them, but don't want to if not necessary.

N3DT
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